The more efficient use of petroleum made necessary by the shortage and increased costs thereof have led, inter alia, to the development of processes which guarantee the optimum possible utilization of this raw material. A considerable proportion of petroleum is converted into heavy and light heating oil and used for generating energy, in particular for producing heat and electricity. The efficiency which can be achieved depends on the completeness of the combustion of the fuel. Various measures which effect complete combustion, i.e. avoid the formation of carbon black, are therefore employed.
One way of achieving this aim is to add certain substances to the heating oil which promote its combustion. Sulfonates and naphthenates of various metals have been known for a long time as additives which suppress the formation of carbon black (cf, for example, J. Vaerman, Journal of the Institute of Petroleum, Volume 50, No. 487 (1964), pages 155-168). Suspended inorganic metal salts and metal oxides also have a marked combustion-promoting effect (cf P. J. Agius et al., 8th World Petroleum Congress Proceedings 5, 27-33 (1971) but, like metal chelates, acetylacetonates, and ferrocene, they have the disadvantage that they readily separate out from the suspension.
Iron and manganese salts of aliphatic carboxylic acids having 10 to 30 carbon atoms are known (from French Patent 1,381,150) as additives which improve the combustion of liquid fuels. Finally, the use of iron salts and/or manganese salts of aliphatic carboxylic acids having 6 to 8 carbon atoms as combustion auxiliaries is described in DE 30 44,907 C2. These compounds have the advantage of being easily oil-soluble and non-toxic, and moreover readily available.
The formation of carbon black is indeed suppressed completely or almost completely during combustion of liquid fuels by using the above-mentioned additives. However, these contribute nothing towards solving the sulfur problem. Both heavy and light heating oils contain greater or lesser proportions of bonded sulfur, depending on their origin. This sulfur is partly burned in the flame to form SO.sub.3, vanadium compounds inter alia likewise contained in the heating oils acting as catalysts. Moreover, SO.sub.3 forms corrosive sulfuric acid in the presence of water. To avoid damage to the combustion equipment, the waste gases must therefore be heated above the dew point of the acid in order to prevent condensation of the acid.
Although major damage from corrosion can be excluded by construction measures, because of the diversity of consumers with their different furnaces, a different route is taken in practice. It has been found that the formation of sulfuric acid can be avoided by addition of magnesium in the form of a compound which is dissolved in the heating oils. The magnesium oxide formed during combustion of the magnesium compound deactivates the substances which catalyze the oxidation of the sulfur to SO.sub.3 and reacts with sulfuric acid to give magnesium sulfate. This compound is deposited as a protective dust on the components of the combustion device and, moreover, does not pollute the environment if it is released into the atmosphere.
The development of carbon black and the formation of SO.sub.3 during combustion of heating oils can, therefore, be drastically reduced by addition of oil-soluble iron and magnesium compounds. This results in an increase in the economic efficiency of the combustion of the heating oil; since the carbon content of the oil is utilized more completely, the temperature of the combustion gases can be lowered, and the sulfuric acid liberated can cause damage neither to the combustion devices nor the environment.
The iron and magnesium salts of aliphatic carboxylic acids used as additives for heating oils are prepared separately. A water-soluble iron salt, for example the nitrate, is usually employed as the starting material and is reacted with an aliphatic carboxylic acid in the presence of an alkali metal hydroxide. The magnesium salt is obtained by reaction of magnesium oxide or magnesium carbonate with the carboxylic acid at temperatures above 120.degree. C. In spite of the high temperatures, the reaction time is at least 2 hours. The use of such high temperatures leads to caking in the reactor, which impedes uniform reaction of the magnesium oxide and carboxylic acid and thus leads to impure products and makes emptying of the reactor difficult.